Sound recording



Patented Sept. 27, 1938 UNITED STATES SOUND RECORDING William A. Mueller, Hollywood, alif.,'assignor to United Research Corporation, Burbank, Calif., a corporation of Delaware Application February 10, 1936, Serial No. 63,087

1 Claim. (01. 179 1o0.3)

This invention relates to the recording of sounds from a plurality of sources in such a manner that the various sounds form a pleasing tone balance therebetween, and more particularly 5 to an improvement in the duping arrangement disclosed and claimed in the co-pending application of George R. Groves, Serial No. 63,079, filed Feb. 10, 1936. That application discloses a method of duping both background sounds and main dialog sounds upon a single record. In the original recording the background sounds are recorded upon a duping record while compressing the amplitude thereof, so as to obtain a compressed record wherein the amplitude level range is sumciently low to prevent any instantaneous high levels from rising high enough to interfere with the level of the dialog to be mixed therewith. The compressed background record and a second dialog record are combined in a final duping operation to obtain the master rec- The above mentioned method is useful in cases where a compressed background sound effects record is obtainable, however most moving picture studios at the present time retain sound record libraries, containing standardsound effects records in which the volume level thereof has not been automatically compressed. Furthermore, background sound eifects may be recorded in various parts of the world where such compression recording equipment is not available.

One object of the present invention is to employ a standard uncompressed sound effects rec- 0rd in a duping operation wherein in the final duped record the sound effects are substantially compressed. This is accomplished by employing a. duping circuit for both standard uncompressed dialog and standard uncompressed sound effects records, and providing in the sound effects recording channel a volume compressor circuit whereby the sound effects currents are compressed in amplitude before they are mixed with the dialog sound currents. That is, compression of the sound effects currents is such that the higher amplitudes are compressed to a greater degree than the lower amplitudes under control of their own variations in amplitude.

For further details of the invention, reference may be had to the accompanying drawing whereume compressor circuit adapted to be employed in the duping channel illustrated in Fig. 1,

Referring in detail to Fig. 1, the dialog or primary sound record. "I, and sound effects or secondary Isound records H and I2 are all of 5 the unco-mpressed volume type. These records Ill, H and I2 are placed in reproducing "apparatuses l3, l4 and I5 respectively, each of whichcomprises an exciter lamp I6, and lens and slit combination I! for focusing a beam of 10 light from the exciter lamp [6 upon'the sound film record in the form of anarrow horizontal slit of light. The beam of light modulated by the film is allowed to fall upon a suitable photoelectric cell 29, 2| M22. The speech currents .15 generated in the photo-electric cell. 2|! in'accordan'ce with the dialog record l0, are'pas'sed through a preamplifier 23, a manually operable potentiometer 24 and thence into'a common mixer .bus 25. The sound currents corresponding 20 to the sound effects record II, as generatedin the photo-electric cell 2| are passed througha pre-amplifier 26, a manually operable potentioni eter 21, and thenc'einto a volume compressor circuit generally indicatedat 28. Here the ampli- 25 tude of the currents are automatically controlled, that is, the lower :level amplitudes are increased to a greater-degree than the higher "level amplitudes. The distorted currents are then fed into the common mixer bus 25 where they combine 30 with the uncompressed dialog sound currents from the photo-electric cell 20. Preferably an isolating amplifier 29 is inserted in the circuit between the volume compressor circuit 28 and the mixer bus 25 to prevent the amplified speech currents from the photo-electric cells 20 or 22 from affecting the operation of the compressor circuit 28. The second background sound effects record l2 may also be introduced into the duping channel by closing a switch 54. A volume 40 compressor circuit similar to that of 28, is employed for record l2 and may be adjusted for a different degree of compression than that of compressor 28. Thus, if it is desired to make the sound effects of the record [2 more predominant 45 in the final duped record, the compressor 55 is so adjusted as to allow a greater volume level range of the sound effects current than that of 28. An isolating amplifier 56, similar to that of 29 and for the same purpose, is preferably em- 50 ployed between the compressor circuit 55 and the common mixer bus 25. The combined sound currents from mixer bus 25 pass through a channel amplifier 30, a manually operable channel potentiometer 3| and then into a suitable sound 55 recorder for the master film 32, such as a light valve 33. Preferably, before passing into the recorder 33, the speech currents are fed through a noise reduction circuit 34 of the standard shuttering or biasing type in order to produce a variable area or variable density noise reduction sound track upon film 32. However, the film compression-expansion type of recording may be employed here by introducing a film compressor circuit in the duping channel in place of the noise reduction circuit 34. In this case the duped record 32 would be of the compressed type and may be reproduced in any sound reproducer employing a volume expansion system such as that disclosed and claimed in the co-pending application of Burton F. Miller, Ser. No. 42,116, filed Sept. 25, 1935. In such a circuit the higher level amplitudes are increased to a greater degree than the lower level amplitudes of the compressed record and by an amount substantially equal to the amount of compression originally obtained, thus the reproduced sound would be substantially the same as the sound originally recorded.

Referring now to Fig. 2, which is a circuit diagram of a compressor circuit adapted to be employed in the duping channel Fig. 1, the input 35 is coupled through an input transformer 36 to the input of a pair of impedance tubes 31 and 38. Tubes 31 and 38 have their anode circuits coupled to an output transformer 40. A pair of leads 4| are shunted across the output 42 of transformer 40 and fed a portion of the sound currents passing through the compressor circuit into an amplifier 43, a rectifier 44 and a rectifier filter 45. The output terminals of filter 45 are connected. in series in a common lead of the gridcathode circuits of tubes 31 and 38. A static negative grid potential is applied to the grids of tubes 31 and 38 through a drop in potential across a resistance 46 which is placed in series in a direct current supply generally indicated at 41. The polarity of the output terminals of the filter 45 is so arranged that at higher amplitude levels, the negative grid bias is increased, thus decreasing the final output levels of these higher amplitudes.

In order to prevent the impedance of tubes 31 and 38 from becoming too large, a resistance 48 of approximately 100,000 ohms and a condenser 49 in series is shunted across the cathode-anode circuit of tube 31. A resistance 58 of a similar value to that of 48 and condenser in series is shunted across the other tube 38. In order to prevent the impedance of tubes 31 and 38 from dropping to an extremely low value, a resistance 52 of about 100,000 ohms is provided in series in the grid cathode circuits of tubes 31 and 38. By the use of such resistance 52 any tendency for the grids of tubes 31 and 38 to be driven positive by high signal levels will be counteracted by the drop in potential across this resistance 52 due to the grid current flow.

Although the above description and accompanying drawing have described and illustrated the invention in conjunction with the sound on film type of sound record, it is to be understood that any type of sound record may be employed, such as wax, wire, etc., without departing from the scope of the invention.

Having thus described the invention what is claimed as new and desired to secure by Letters Patent is:

An apparatus for re-recording from a source of primary sound and from a source of secondary sound upon a standard noise reduction sound record comprising means for generating electrical currents corresponding to said secondary sound, means for generating electrical currents corresponding to said primary sound, means for varying the amplitudes of the currents corresponding to said primary sound in a substantially constant ratio, means for varying the amplitudes of the currents corresponding to said secondary sound, said variations being such that the higher amplitudes are varied in a ratio different from said lower amplitudes, means for adjusting the absolute amplitudes of said high level secondary currents to have a non-interfering relationship with the currents of said primary sound, means for combining the primary sound currents and the secondary sound currents in a common channel, and means for recording said combined currents into a standard noise reduction record.

WILLIAM A. MUELLER. 

